The relationship between immune reactivity and Recurrent Aphthous Ulcers (RAU) will be studied in individuals with RAU. Subjects will be selected from a previously studied population of student nurses and from patients referred to the Oral Medicine services at the University Hospital and the Philadelphia General Hospital. Immune reactivity will be studied by in vitro techniques measuring lymphocyte cytotoxicity by measuring the release of radiolabelled compounds from oral epithelial cell cultures taken from RAU positive and RAU negative individuals. Data will be gathered in order to determine the cell types involved in the cytotoxic reaction and to correlate in vitro data with clinical aspects of the disease.